1. Field of the Invention
The present invention relates to a single or multi-stage centrifugal pump which is designed so that axial thrust is hydraulically balanced by means of a balancing disc device. More particularly, the present invention pertains to a centrifugal pump which uses both the above-described balancing disc device and a magnetic thrust bearing.
2. Prior Art
Balancing disc devices have heretofore for a long time been employed as axial thrust balancing devices in centrifugal pumps. A typical conventional balancing disc device in a centrifugal pump is arranged such that a fixed throttle constituted by an axial gap, and a movable throttle constituted by a radial gap, are disposed in series in a passage for returning a part of the pressurized water from the discharge stage of the pump to a low-pressure section to thereby vary the pressure inside the space between the two throttles in accordance with a change in the flow resistance at the movable throttle caused by a change in the axial position of the pump shaft, thereby automatically balancing the axial thrust in the pump. It has already been confirmed by many years of achievement that the axial thrust balancing function of such conventional balancing devices, if properly designed, is substantially perfect during steady-state running of the pumps, although there are changes and modifications in regard to the details of the structure, that is, regarding the positional relationship between the fixed and movable throttles and as to which one of them is disposed closer to the center of the pump rotating shaft.
FIG. 1 is a fragmentary sectional view of one example of a multi-stage centrifugal pump to which the above-described balancing disc type axial thrust balancing device is applied. A balancing disc 3 is formed so as to be integral with the reverse surface of a main shroud 2a of a final-stage impeller 2 attached to a pump (motor) shaft 1. A balancing disc seat 4 is attached to a motor casing in opposing relation to the balancing disc 3 with a radial gap .epsilon..sub.2 provided therebetween. An axial gap .epsilon..sub.1 is provided between a liner ring integally formed on the reverse surface of the main shroud and a cylindrical bore formed in the motor casing. Thus, an intermediate chamber 6 is defined between the gaps .epsilon..sub.1 and .epsilon..sub.2. The rear side (downstream side) of the balancing disc seat 4 is communicated with a motor chamber 8 through a relief passage 7.
During the running of the pump, a part of the fluid discharged from the final-stage impeller 2 flows into the intermediate chamber 6 through the axial fixed gap .epsilon..sub.1 that is formed at the reverse side of the main shroud 2a so that the size of the gap .epsilon..sub.1 is fixed, thus applying leftward (i.e., toward the suction port) axial thrust to the impeller 2. The leftward axial thrust causes the shaft 1 to move leftward against the rightward thrust acting on a shroud 2b. In consequence, the size of the radial gap .epsilon..sub.2 that constitutes a variable throttle, increases, so that the flow resistance at the variable throttle is lowered. However, since the gap .epsilon..sub.1, that is present at the upstream side of the intermediate chamber 6, constitutes a fixed throttle where the flow resistance is not changed by the sideward movement of the shaft 1, as the gap .epsilon..sub.2 enlarges, the pressure inside the intermediate chamber 6 is lowered. As a result, rightward thrust acts on the impeller 2 as a whole, causing the gap .epsilon..sub.2 to be narrowed. As the gap .epsilon..sub.2 narrows, the pressure inside the intermediate chamber 6 rises again and the rightward thrust acting on the impeller 2 decreases, resulting in the shaft 1 being moved leftward again.
Thus, the balancing disc device constitutes an automatic control system which determines the size of the gap .epsilon..sub.2 by itself in accordance with the level of the axial thrust generated by the impeller. It should be noted that if the positional relationship between the gaps .epsilon..sub.1 and .epsilon..sub.2 is opposite to the above, the movement of the shaft 1 and the change in the size of the gap .epsilon..sub.2 are reversed to the above; however, the operating principle and the effectiveness are the same as the above.
As has been described above, in the conventional axial thrust balancing device, the size of the variable gap .epsilon..sub.2 is automatically set at an optimal value during steady-state running and the pump is run smoothly if the radial (annular) area of the intermediate chamber 6 is set so as to be a proper size relative to the annular area defined between the impeller mouth ring 2c, and the shaft 1, and the size of the fixed gap .epsilon..sub.1 is set at a proper value in connection with the annular area of the intermediate chamber 6. Accordingly, pumps which are equipped with a balancing disc device do not usually have a thrust bearing for fixing the pump shaft in the axial direction.
However, in the conventional type device, when the pump is in a transient running state, for example, at the time of starting or stopping the pump, there is a time lag between the formation of the differential pressure between the upstream and downstream sides of the impeller that causes axial thrust, and the formation of the differential pressure between the upstream and downstream sides of the balancing disc device that causes a thrust counter to the axial thrust. This is because each differential pressure is formed by a pressure drop in the fluid upon passing through a throttle passage and therefore a certain time is needed for the flow to become steady. For this reason, the impeller thrust and the balancing disc thrust are not in balance with each other in the transient running state, although such an imbalanced state continues only for an extremely short period of time. Therefore, there has heretofore been a fear that the gap .epsilon..sub.2 will momentarily become zero, which results in contact between metallic members and, if this is repeated, the metallic members may become worn. Further, in the case of a vertical shaft type pump, there has been a problem that the rotator cannot be fixed to a certain axial position when the pump is suspended, which has been inconvenient.
For the above-described reasons, it is an occasional practice to use a thrust bearing in conjunction with a balancing disc device. In such a case, the value of the gap .epsilon..sub.2, during the running of the pump is, estimated by precise calculation and the position of the thrust bearing is set so that the estimated value for the gap .epsilon..sub.2 is maintained. Accordingly, a great deal of time and labor is required, and if there is a mistake in these procedures, the load applied to the thrust bearing may be excessive, resulting in the bearing becoming worn prematurely.